Yes, intarsia will work for this sweater. The other possibility is to knit it side to side. You could knit from the left front, across the back to the right front. You'd have to knit the sleeves separately and also knit them side to side. The advantage is that you would only have to work with one yarn color at a time. This pattern might get you started:http://www.freepatterns.com/detail.h...210&cat_id=378
Very creative design from your grandson by the way.